Plant 265

Derris elliptica (Wall.) Benth. (Fabaceae)

Derris

Derris is a small genus of vines that is found in the Old World tropics, especially South East Asia. The small, pink flower clusters of Derris elliptica, with the distinctive shape characteristic of peas and beans, are followed by leathery seedpods. However, flowers and fruits are rare in cultivation.

The genus is best known as a source of rotenone, a colourless, odourless compound that is extremely toxic to fish and invertebrates. However, rotenone is not unique to Derris. Rotenone's toxicity means Derris species are used as both piscicides and insecticides.

As an aid to fishing and fish farming, derris roots are pounded to release rotenone, and then thrown into the water. Dead or stunned fish float to the surface where they can be easily gathered up. In farming, the same method is used to clear ponds of existing predatory fish before introducing cultivated species. The small quantities of rotenone that persist in the edible parts of the fish are degraded by cooking.

In Europe and America, powdered derris root was widely used as an insecticide by both amateur and professional growers from the nineteenth century. The non-selectivity of rotenone makes it effective against many garden and horticultural pests, including greenfly, sawfly, caterpillars, thrips, small beetles and spider mite.

However, rotenone is toxic to mammals and large doses may be fatal to humans, although this is rare and the World Health Organisation classes it as only moderately harmful. Small quantities are effectively broken down by the human liver, and when used as a pesticide rotenone persists on the crop for only a few days. For these reasons, it was considered a safe treatment for edible crops near to harvest date.

Rotenone was one of only four pesticides approved for use in organic farming by the Soil Association, which assesses substances based on their origin, environmental impact and persistence as residues. Today, the use of derris as a pesticide has dramatically declined from former levels, possibly due to studies that link it with Parkinson's disease. Official approval for the use of derris was withdrawn in 2008, meaning it is illegal to store or use it as a pesticide in the UK.

Plants have evolved sophisticated chemistries to protect themselves from predators, not for human convenience. Derris is a reminder that, despite a popular belief that organically derived substances are inherently less harmful than synthetic ones, the true picture can be more complex.